Crude oil in different oil reserves or even in the same earth formation can have different chemical compositions. For example, different crude oils can have different chemical compositions such as different relative concentrations of methane, ethane, propane, butane, pentane, hexane, heptane and the like. Petroleum analysts and engineers would like to know the chemical compositions of oil reserves in order to efficiently recover and process the oil that they find. Gas chromatography (GC) is the standard surface laboratory method for obtaining such detailed chemical composition but retention times are very dependent upon temperature and carrier-gas flow rates and it would be extremely cumbersome to implement GC downhole, which would require a very large and costly engineering effort. Hence, it would be appreciated in the oil industry if new and efficient methods were developed to estimate a chemical composition of oil downhole in real time.